In a background art, in a vehicular seat, there is known a constitution of arranging a reclining apparatus operable to adjust a backrest angle of a seat back. Here, JP-A-2005-83535 discloses a specific constitution of the above-described reclining apparatus. According to the disclosed reclining apparatus, an outer gear is disposed in a state of meshing with an inner gear, and the backrest angle of the seat back is varied by a relative revolving movement of the outer gear along an inner peripheral tooth face of the inner gear.
Further, according to the disclosure, a tooth shape of the above-described inner gear or outer gear is formed to have a shape of a cycloid curve or a trochoid curve. For this reason, a meshing line of the two gears is set to a shape of a circular arc, and therefore, in comparison with a gear train of a publicly-known tooth shape having an involute curve resulting a linear meshing line, a meshing strength can be increased by the increase in the meshing rate.